Internet Control Message Protocol

Internet Control Message Protocol (ICMP) defined by RFC 792 and RFC 1122 is used for network error reporting and generating messages that require attention. The errors reported by ICMP are generally related to datagram processing. ICMP only reports errors involving fragment 0 of any fragmented datagrams. The IP, UDP or TCP layer will usually take action based on ICMP messages. ICMP generally belongs to the IP layer of TCP/IP but relies on IP for support at the network layer. ICMP messages are encapsulated inside IP datagrams.

ICMP will report the following network information:

Timeouts
Network congestion
Network errors such as an unreachable host or network.

The ping command is also supported by ICMP, and this can be used to debug network problems.

ICMP Messages:

The ICMP message consists of an 8 bit type, an 8 bit code, an 8 bit checksum, and contents which vary depending on code and type. The below table is a list of ICMP messages showing the type and code of the messages and their meanings.

Type	Codes	Description	Purpose
0	0	Echo reply	Query
3	0	Network Unreachable	Error
3	1	Host Unreachable	Error
3	2	Protocol Unreachable	Error
3	3	Protocol Unreachable	Error
3	4	Fragmentation needed with don't fragment bit set	Error
3	5	Source route failed	Error
3	6	Destination network unknown	Error
3	7	Destination host unknown	Error
3	8	Source host isolated	Error
3	9	Destination network administratively prohibited	Error
3	10	Destination host administratively prohibited	Error
3	11	Network Unreachable for TOS	Error
3	12	Host Unreachable for TOS	Error
3	13	Communication administratively prohibited by filtering	Error
3	14	Host precedence violation	Error
3	15	Precedence cutoff in effect	Error
4	0	Source quench	Error
5	0	Redirect for network	Error
5	1	Redirect for host	Error
5	2	Redirect for type of service and network	Error
5	3	Redirect for type of service and host	Error
8	0	Echo request	Query
9	0	Normal router advertisement	Query
9	16	Router does not route common traffic	Query
10	0	Router Solicitation	Query
11	0	Time to live is zero during transit	Error
11	1	Time to live is zero during reassembly	Error
12	0	IP header bad	Error
12	1	Required option missing	Error
12	2	Bad length	Error
13	0	Timestamp request	Query
14	0	Timestamp reply	Query
15	0	Information request	Query
16	0	Information reply	Query
17	0	Address mask request	Query
18	0	Address mask request	Query

ICMP is used for many different functions, the most important of which is error reporting. Some of these are "port unreachable", "host unreachable", "network unreachable", "destination network unknown", and "destination host unknown". Some not related to errors are:

Timestamp request and reply allows one system to ask another one for the current time.
Address mask and reply is used by a diskless workstation to get its subnet mask at boot time.
Echo request and echo reply is used by the ping program to test to see if another unit will respond.